The incidence and etiology of aplastic anemia in Thailand will be investigated in a population-based case-control study. This includes an extension of an existing study in Bangkok and its expansion to two rural areas of Thailand, Khonkan and Songkla. All hospitals with at least 100 beds in Bangkok and its suburbs (population, 8.7 million) are already participating in the study. In the rural areas (total population, 9.6 million) all hospitals with at least 100 beds will be enrolled. Cases will be identified prospectively, and reviewed independently by a committee of hematologists to confirm the diagnoses. Suitable controls will be selected from among other patients in the same hospitals (e.g., patients with trauma and other acute conditions) and matched to cases for age, sex, and the time of hospitalization. All identified cases and controls will be interviewed to obtain information on drug use, occupational and chemical exposures, personal data, and relevant medical history. Serum and DNA samples will be collected from all cages and one control per case for analysis of hepatitis, Epstein-Barr, and other viruses. At the end of the study, there will be 330 cases and 1320 controls available for analysis. Incidence rates and the risk of aplastic anemia among individuals exposed to various factors will be estimated. Because of the population-based design, absolute as well as relative measures of risk will be provided. The preliminary results from the existing study suggest that aplastic anemia is, overall, overall, twice as common in Bangkok as in Western countries, and that it is much more common among young individuals. This is possibly due to environmental factors. An extension of the study will provide more information about the role of the drugs and other factors, especially household pesticides and various occupational exposures; expansion to the rural areas will make possible the evaluation of the role of pesticides used in agriculture, and other factors unique to rural Thailand.